How to set gradient transparent mask for UIVIew.layer. its needs for create scroll fade animation effect.

FadeMask

- (void)viewDidLoad
{
    [super viewDidLoad];
    UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"Screen Shot 2013-07-23 at 5.29.05 PM.png"]];
    UIView *viewWithMask = [[UIView alloc] initWithFrame:CGRectMake(0, 200, self.view.frame.size.width, 100)];
    [viewWithMask setBackgroundColor:[UIColor greenColor]];
    // Recreate gradient mask with new fade length
    CAGradientLayer *gradientMask = [CAGradientLayer layer];
    gradientMask.bounds = viewWithMask.layer.bounds;
    gradientMask.position = CGPointMake(CGRectGetMidX(viewWithMask.bounds), CGRectGetMidY(viewWithMask.bounds));
    gradientMask.shouldRasterize = YES;
    gradientMask.rasterizationScale = [UIScreen mainScreen].scale;
    gradientMask.startPoint = CGPointMake(0.0, CGRectGetMidY(viewWithMask.frame));
    gradientMask.endPoint = CGPointMake(1.0, CGRectGetMidY(viewWithMask.frame));
    // setup fade mask colors and location
    id transparent = (id)[UIColor clearColor].CGColor;
    id opaque = (id)[UIColor blackColor].CGColor;
    gradientMask.colors = @[transparent, opaque, opaque, transparent];
    // calcluate fade
    CGFloat fadePoint = 50 / CGRectGetWidth(viewWithMask.bounds);
    NSNumber *leftFadePoint = @(fadePoint);
    NSNumber *rightFadePoint = @(1 - fadePoint);
    // apply calculations to mask
    gradientMask.locations = @[@0, leftFadePoint, rightFadePoint, @1];
    viewWithMask.layer.mask = gradientMask;
    [viewWithMask.layer setCornerRadius:50];
    [viewWithMask.layer setMasksToBounds:NO];
    [viewWithMask addSubview:imageView];

    [self.view addSubview:viewWithMask];
}